Already famous for his speech at the 2004 Democratic National Convention, Barack Obama entered the Senate with more than the usual aspirations about the impact he could have.

So in 2005, he had his office arrange informal seminars so that experts on health care, the economy, energy and education could brief him. "I'm not running for president," he told a group of experts at his Capitol Hill office in the spring of 2006. But he said he had a "national voice" and wanted to use it.

When Obama changed his mind and decided to run for president after only two years in the Senate, however, he effectively dismissed the importance of policy proposals, declaring in one speech in early 2007, "We've had plenty of plans, Democrats," and in another: "Every four years, somebody trots out a white paper, they post it on the Web." He cast his "new kind of politics" in terms of his ability to transcend divisions and his unique biography and offered few differences on issues from Sen. Hillary Rodham Clinton and the other Democratic presidential candidates.

But now this approach faces a new test from Sen. John McCain. The GOP candidate is making an aggressive appeal to independents by emphasizing his past and present stances against party orthodoxy, particularly his proposals to combat global warming.

Obama has not emphasized any signature domestic issue, or signaled that he would take his party in a specific direction on policy, as Bill Clinton did with his "New Democrat" proposals in 1992 that emphasized welfare reform or as George W. Bush did with his "compassionate conservatism" in 2000, when he called on Republicans to focus more on issues such as education.

Obama's campaign is "clearly politically transformative, it's clearly from a policy standpoint been cautious," said James K. Galbraith, a liberal activist and economist at the University of Texas at Austin who had backed former senator John Edwards in the early primaries.

"The change that Senator Obama has promised is one of tone and leadership style," said William A. Galston, who was a domestic policy adviser to President Bill Clinton and is backing Sen. Clinton but who said he would enthusiastically support Obama if he is the party's nominee. "He has not dissented from party orthodoxy in the way Bill Clinton did on the way to the presidency in 1992," Galston added.

Heather Higginbottom, who runs Obama's policy office at the campaign's Chicago headquarters, cited education as one area in which Obama offers ideas that are not traditionally Democratic, arguing that the problem is not all about schools or funding, but about parents who let their children watch too much television. She said his proposal to give teachers bonus pay if they receive special training or if their students score high on standardized tests is an idea that some liberal-leaning teachers unions oppose. And she said the campaign has brought "fresh thinking" on many issues, particularly on one of Obama's favorites: increased government transparency.

But Higginbottom said the campaign's emphasis is on practical solutions, not ideological points. "I know it's interesting from a political perspective to look left, right and center, but we want to put forward ideas that will move forward in Congress," she said. "And we have the potential to engage people in a way they haven't been engaged recently and give them the tools to participate."

David Axelrod, Obama's top political adviser, said that the campaign will devote more staff members to policy (there are now seven) and that the senator's speeches will increasingly highlight his proposals.

"The next six months is going to be about competing visions for this country," he said. "Obama is looking forward, and his policies will reflect that."

Obama's domestic policy proposals, including expanding health care to all Americans and offering tax cuts for the middle class while raising taxes for those who make more than $250,000 a year, differ little from those that Clinton and other Democrats have proposed during the primaries. His ideas for solving the nation's housing crisis are similar to those of congressional Democrats, offering aid to people who cannot pay their mortgages and proposing a second economic stimulus package.

Obama, like many congressional Democrats, has pushed for more education funding, a cap-and-trade system to reduce greenhouse-gas emissions, and immigration legislation that would create a path to citizenship for people who are now in the United States illegally.

In part, Obama's approach reflects the broad consensus that has developed during the Democratic primaries. Unlike Republicans -- many of whom disagree with McCain on issues such as global warming and immigration -- Democratic presidential candidates, the party's leaders in Congress and Democratic voters largely agree on an agenda. There is little of the left-center divide of the Bill Clinton era. Self-identified independent voters broadly favored the Democrats' approach over that of the GOP on Iraq, health care, the economy and dealing with the federal budget deficit, according to a recent Washington Post poll.

Jared Bernstein, a liberal economist at the Economic Policy Institute, praised him for offering a more progressive agenda than the past two Democratic presidential nominees, former vice president Al Gore and Sen. John F. Kerry, neither of whom proposed a universal health-care plan, as Obama has. "There's a recognition that small-bore approaches to solving the big challenges is not sufficient," Bernstein said.

Obama's policy ideas reflect the group of mainstream Democratic advisers he has surrounded himself with, many of them younger colleagues of experts who had held top-level positions in the Clinton administration and ended up working for the former first lady's campaign.

Jeffrey B. Liebman, a Harvard economist who advises Obama on budget issues, had been a top aide to Gene Sperling, President Clinton's top economics adviser, who now works for Sen. Clinton's campaign; Michael Froman, a Citigroup executive who advises Obama on Wall Street issues, was a top aide to former Treasury secretary Robert Rubin, another Clinton backer.

Obama also brought in people who were not in the Clinton orbit, but most of them are not new to Washington. The advisers say that what drew them to Obama was not his embrace of their policy views but rather his potential appeal for getting things done.

"His message of uniting people and trying to do something new was apparent from the beginning, and so he sort of had me at hello," said University of Chicago professor Austan Goolsbee, Obama's top economic adviser.

The campaign, however, has distanced itself from more controversial views, such as Goolsbee's description of Obama's opposition to the North American Free Trade Agreement as political rhetoric. Goolsbee denied the account of what he said at a meeting at the Canadian consulate in Chicago, and Obama has kept his anti-NAFTA rhetoric.

Looking forward, Galston said that while Obama and McCain would each seek to emphasize independence from their parties on "second-tier" issues such as teacher pay or global warming, on the big issues, such as the Iraq war and the economy, they would hew to party orthodoxy while at the same time arguing that the other is even more tied to his own party.

"The Obama campaign will argue on those issues McCain is, if anything, more conservative than Bush," Galston said. "The McCain campaign will argue although Senator Obama has campaigned on a promise to bring us back together, that in fact he is not a moderate, despite his tone, but is a liberal."

Bruce Reed, who also was a policy adviser to President Clinton and now supports Sen. Clinton, said it is important for the eventual Democratic nominee to show some break from the party, to burnish centrist credentials. "Our candidate will need as many proof points as possible that we're not the weak-on-defense, big-spending liberal the Republicans always say they are," he said.

Obama aides, however, say their approach will work because most voters are looking not for a new vision for expanding health care but rather for a reformed political system such as the one Obama calls for, one that would solve problems rather than resort to bickering.

Galston said Obama's approach could succeed in a general-election campaign as long as the candidate made sure voters were more familiar with his plans, but he was more skeptical about the approach working if Obama is elected president. "There are many scholars . . . who believe that polarization in the country between the parties is pretty thorough and that a change in tone may not be sufficient," he said.